The automotive, aerospace, consumer appliance, and other industries require adhesives that have a combination of properties such as high durability, resistance to continuous exposure to high temperatures and good adhesion to a wide range of substrates. Epoxy resin adhesives have often been used in the form of liquid adhesives for these applications requiring structural and semi-structural bonding. Several difficulties occur when a liquid adhesive is used, such as evaporation of solvent, loss of adhesive from the required bonding area and uneven distribution of adhesive over the surfaces to be bonded. Use of adhesives in film form eliminates difficulties which may occur when a liquid adhesive is used.
One conventional method of producing film adhesives involves dissolving a solid resin composition in a volatile organic solvent, pouring the resulting solution onto a flat surface and then evaporating the solvent to leave a film of the resin composition. In another conventional method of making a film adhesive, a solid resin composition is heated to its softening point, extruded through a slit and then cooled to form a solid film. These conventional methods have associated disadvantages. Use of a volatile solvent usually gives rise to toxicity and/or flammability problems, while evaporation of the solvent has expensive energy requirements and solvent recovery equipment is needed if costly wastage is to be avoided. Heating, whether for evaporation of the solvent or to soften the solid composition prior to extrusion, often results in advancement (polymerization) of the resin, shortening the shelf life of the resulting film adhesive and leading to its premature cure.